Activated carbon, a porous adsorbent, is widely used in industry in the purification of liquids and gases. For example, a gas which is to be purified is passed through a bed of granular activated carbon. As the gas passes through the activated carbon bed, molecules of impurities in the gas are adsorbed onto the surface of the activated carbon. Consequently, the larger the surface area of the activated carbon the more efficient the filter will be in removing impurities.
Activated carbons of commercial importance can exhibit specific surface areas of up to 1500m.sup.2 /g contained in a pore volume of approximately 1ml/g and to achieve this large surface area, pores of very small dimensions are involved. As pore size increases the surface area per unit volume decreases.
Pore sizes are defined as micropores, mesopores and macropores. Micropores and mesopores contribute to the adsorptive capacity of an activated carbon whereas the macropores, containing very little of the total surface area, do not contribute to the adsorptive capacity of the activated carbon.
Therefore, the pore structure of the carbon with its inherent surface area is of paramount importance in determining the effectiveness of the activated carbon as an adsorbent.
However, in the case of granular activated carbon the density is also an important feature of the effectiveness of the adsorbent, as the application of granular activated carbon is invariably in the form of a static bed of fixed volumetric size.
Chemically activated carbon, by virtue of its raw materials and manufacturing process, tends to be of low density with a highly developed mesopore structure. The latter feature is a desirable feature, the former a disadvantage of any granular form of chemically activated carbon. The success of any process to manufacture granular chemically activated carbon is dictated by its capacity to combine retention of the mesoporous nature with development of a high density by minimisation of macroporosity, which does not contribute to adsorptive effectiveness.
The normal method used to determine the efficiency of a granular activated carbon is the weight of material it can adsorb, per unit volume of activated carbon.
This test is normally carried out by placing a volume of activated carbon in a standard U-tube and passing a vapour through the activated carbon. The carbon is weighed before and after this process and the difference provides the weight of substance adsorbed by the carbon.
The raw material normally used in the production of chemically activated carbon is a carbonaceous vegetable material such as wood which has been milled to a 2-5mm particle size. The activated carbon when produced is usually ground into a powder form for use in liquid purification or can be shaped into pellets of various sizes using a binder, for use in gas purification.
There are a number of such uses for activated carbons from the removal of coloured compounds present as impurities in the products of a chemical reaction to the purification of gases prior to discharge to the atmosphere. However, there are a number of problems inherent in the use of wood as a raw material to produce directly a chemically activated pelletised granular form.
The hollow fibrous structure of wood is such that it is impossible to produce a high density granular activated carbon from a wood flour raw material. The wood also lacks a natural binding agent, such as lignin, in sufficiently large quantities and an additional binding agent would have to be introduced in the production of the activated carbon to prevent the breakdown of the particle structure of the granular carbon during processing.
The cellular structure of wood is such that the granular activated carbon produced from it is capable of adsorbing a maximum of 6-7g of impurities/100ml of activated carbon due to its low density. This is below the figure required for a number of applications of activated carbon.
This is not of great importance in a powder liquid phase application as there is normally no strict limit on the volume of activated carbon which can be used.
However, as previously discussed in the case of granular applications, there is an upper limit on the volume of activated carbon which can be used. Consequently, if the granular activated carbon is to be able to perform effectively then the volumetric adsorption factor (g/100ml) must be increased substantially by increase in the product density.
Therefore, the introduction of a method of producing a more efficient activated carbon would be extremely advantageous.